AlGaN/GaN HFETs with new ohmic and Schottky contacts for thermal stabilityup to 400 degrees C

Results on the thermal stability of AlxGa1- xN/GaN Heterostructure Field-Ef fect Transistors (HFETs) are presented. The epilayer structures were grown by MOVPE on sapphire with Al mole fractions in the AlGaN barrier layers of 20 acid 25%, respectively. Different metallizations for ohmic and Schottky contacts were employed and their influence on the thermal stability of HFET s was investigated. Transistors with standard Ti/Al based ohmic and Pt base d Schottky contacts showed rapid degradation after aging at 400 degrees C. Using a WSiN diffusion barrier on top of the Ti/Al/Ti/Au ohmic contact the morphology could be strongly improved with smooth contact surfaces and well defined contact edges. The ohmic contact resistance of this:system is ther mally stable and exhibits a value of 0.77 Omega mm after aging at 400 degre es C. Transistors with WSiN/Au Schottky gate contacts already showed improv ed aging behaviour while HFETs with Ir/Au Schottky contacts and 25% Al-mole fraction in the barrier layer demonstrated long term stability at 400 degr ees C with the highest Schottky barrier height and lowest gate leakage curr ent.